package org.apache.lucene.util;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.NoSuchElementException;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.WeakHashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Map.Entry;

import org.apache.lucene.analysis.TokenStream; // for javadocs
import org.apache.lucene.analysis.tokenattributes.TermAttribute;
import org.apache.lucene.analysis.tokenattributes.CharTermAttributeImpl;

/**
 * An AttributeSource contains a list of different {@link AttributeImpl}s,
 * and methods to add and get them. There can only be a single instance
 * of an attribute in the same AttributeSource instance. This is ensured
 * by passing in the actual type of the Attribute (Class&lt;Attribute&gt;) to 
 * the {@link #addAttribute(Class)}, which then checks if an instance of
 * that type is already present. If yes, it returns the instance, otherwise
 * it creates a new instance and returns it.
 */
public class AttributeSource {
    /**
     * An AttributeFactory creates instances of {@link AttributeImpl}s.
     */
    public static abstract class AttributeFactory {
        /**
         * returns an {@link AttributeImpl} for the supplied {@link Attribute} interface class.
         */
        public abstract AttributeImpl createAttributeInstance(Class<? extends Attribute> attClass);

        /**
         * This is the default factory that creates {@link AttributeImpl}s using the
         * class name of the supplied {@link Attribute} interface class by appending <code>Impl</code> to it.
         */
        public static final AttributeFactory DEFAULT_ATTRIBUTE_FACTORY = new DefaultAttributeFactory();

        private static final class DefaultAttributeFactory extends AttributeFactory {
            private static final WeakHashMap<Class<? extends Attribute>, WeakReference<Class<? extends AttributeImpl>>> attClassImplMap = new WeakHashMap<Class<? extends Attribute>, WeakReference<Class<? extends AttributeImpl>>>();

            private DefaultAttributeFactory() {
            }

            @Override
            public AttributeImpl createAttributeInstance(Class<? extends Attribute> attClass) {
                try {
                    return getClassForInterface(attClass).newInstance();
                } catch (InstantiationException e) {
                    throw new IllegalArgumentException("Could not instantiate implementing class for " + attClass.getName());
                } catch (IllegalAccessException e) {
                    throw new IllegalArgumentException("Could not instantiate implementing class for " + attClass.getName());
                }
            }

            private static Class<? extends AttributeImpl> getClassForInterface(Class<? extends Attribute> attClass) {
                synchronized (attClassImplMap) {
                    final WeakReference<Class<? extends AttributeImpl>> ref = attClassImplMap.get(attClass);
                    Class<? extends AttributeImpl> clazz = (ref == null) ? null : ref.get();
                    if (clazz == null) {
                        try {
                            // TODO: Remove when TermAttribute is removed!
                            // This is a "sophisticated backwards compatibility hack"
                            // (enforce new impl for this deprecated att):
                            if (TermAttribute.class.equals(attClass)) {
                                clazz = CharTermAttributeImpl.class;
                            } else {
                                clazz = Class.forName(attClass.getName() + "Impl", true, attClass.getClassLoader()).asSubclass(AttributeImpl.class);
                            }
                            attClassImplMap.put(attClass, new WeakReference<Class<? extends AttributeImpl>>(clazz));
                        } catch (ClassNotFoundException e) {
                            throw new IllegalArgumentException("Could not find implementing class for " + attClass.getName());
                        }
                    }
                    return clazz;
                }
            }
        }
    }

    /**
     * This class holds the state of an AttributeSource.
     * @see #captureState
     * @see #restoreState
     */
    public static final class State implements Cloneable {
        AttributeImpl attribute;
        State next;

        @Override
        public Object clone() {
            State clone = new State();
            clone.attribute = (AttributeImpl) attribute.clone();

            if (next != null) {
                clone.next = (State) next.clone();
            }

            return clone;
        }
    }

    // These two maps must always be in sync!!!
    // So they are private, final and read-only from the outside (read-only iterators)
    private final Map<Class<? extends Attribute>, AttributeImpl> attributes;
    private final Map<Class<? extends AttributeImpl>, AttributeImpl> attributeImpls;
    private final State[] currentState;

    private AttributeFactory factory;

    /**
     * An AttributeSource using the default attribute factory {@link AttributeSource.AttributeFactory#DEFAULT_ATTRIBUTE_FACTORY}.
     */
    public AttributeSource() {
        this(AttributeFactory.DEFAULT_ATTRIBUTE_FACTORY);
    }

    /**
     * An AttributeSource that uses the same attributes as the supplied one.
     */
    public AttributeSource(AttributeSource input) {
        if (input == null) {
            throw new IllegalArgumentException("input AttributeSource must not be null");
        }
        this.attributes = input.attributes;
        this.attributeImpls = input.attributeImpls;
        this.currentState = input.currentState;
        this.factory = input.factory;
    }

    /**
     * An AttributeSource using the supplied {@link AttributeFactory} for creating new {@link Attribute} instances.
     */
    public AttributeSource(AttributeFactory factory) {
        this.attributes = new LinkedHashMap<Class<? extends Attribute>, AttributeImpl>();
        this.attributeImpls = new LinkedHashMap<Class<? extends AttributeImpl>, AttributeImpl>();
        this.currentState = new State[1];
        this.factory = factory;
    }

    /**
     * returns the used AttributeFactory.
     */
    public AttributeFactory getAttributeFactory() {
        return this.factory;
    }

    /** Returns a new iterator that iterates the attribute classes
     * in the same order they were added in.
     */
    public Iterator<Class<? extends Attribute>> getAttributeClassesIterator() {
        return Collections.unmodifiableSet(attributes.keySet()).iterator();
    }

    /** Returns a new iterator that iterates all unique Attribute implementations.
     * This iterator may contain less entries that {@link #getAttributeClassesIterator},
     * if one instance implements more than one Attribute interface.
     */
    public Iterator<AttributeImpl> getAttributeImplsIterator() {
        final State initState = getCurrentState();
        if (initState != null) {
            return new Iterator<AttributeImpl>() {
                private State state = initState;

                public void remove() {
                    throw new UnsupportedOperationException();
                }

                public AttributeImpl next() {
                    if (state == null)
                        throw new NoSuchElementException();
                    final AttributeImpl att = state.attribute;
                    state = state.next;
                    return att;
                }

                public boolean hasNext() {
                    return state != null;
                }
            };
        } else {
            return Collections.<AttributeImpl> emptySet().iterator();
        }
    }

    /** a cache that stores all interfaces for known implementation classes for performance (slow reflection) */
    private static final WeakHashMap<Class<? extends AttributeImpl>, LinkedList<WeakReference<Class<? extends Attribute>>>> knownImplClasses = new WeakHashMap<Class<? extends AttributeImpl>, LinkedList<WeakReference<Class<? extends Attribute>>>>();

    static LinkedList<WeakReference<Class<? extends Attribute>>> getAttributeInterfaces(final Class<? extends AttributeImpl> clazz) {
        synchronized (knownImplClasses) {
            LinkedList<WeakReference<Class<? extends Attribute>>> foundInterfaces = knownImplClasses.get(clazz);
            if (foundInterfaces == null) {
                // we have a strong reference to the class instance holding all interfaces in the list (parameter "att"),
                // so all WeakReferences are never evicted by GC
                knownImplClasses.put(clazz, foundInterfaces = new LinkedList<WeakReference<Class<? extends Attribute>>>());
                // find all interfaces that this attribute instance implements
                // and that extend the Attribute interface
                Class<?> actClazz = clazz;
                do {
                    for (Class<?> curInterface : actClazz.getInterfaces()) {
                        if (curInterface != Attribute.class && Attribute.class.isAssignableFrom(curInterface)) {
                            foundInterfaces.add(new WeakReference<Class<? extends Attribute>>(curInterface.asSubclass(Attribute.class)));
                        }
                    }
                    actClazz = actClazz.getSuperclass();
                } while (actClazz != null);
            }
            return foundInterfaces;
        }
    }

    /** <b>Expert:</b> Adds a custom AttributeImpl instance with one or more Attribute interfaces.
     * <p><font color="red"><b>Please note:</b> It is not guaranteed, that <code>att</code> is added to
     * the <code>AttributeSource</code>, because the provided attributes may already exist.
     * You should always retrieve the wanted attributes using {@link #getAttribute} after adding
     * with this method and cast to your class.
     * The recommended way to use custom implementations is using an {@link AttributeFactory}.
     * </font></p>
     */
    public void addAttributeImpl(final AttributeImpl att) {
        final Class<? extends AttributeImpl> clazz = att.getClass();
        if (attributeImpls.containsKey(clazz))
            return;
        final LinkedList<WeakReference<Class<? extends Attribute>>> foundInterfaces = getAttributeInterfaces(clazz);

        // add all interfaces of this AttributeImpl to the maps
        for (WeakReference<Class<? extends Attribute>> curInterfaceRef : foundInterfaces) {
            final Class<? extends Attribute> curInterface = curInterfaceRef.get();
            assert (curInterface != null) : "We have a strong reference on the class holding the interfaces, so they should never get evicted";
            // Attribute is a superclass of this interface
            if (!attributes.containsKey(curInterface)) {
                // invalidate state to force recomputation in captureState()
                this.currentState[0] = null;
                attributes.put(curInterface, att);
                attributeImpls.put(clazz, att);
            }
        }
    }

    /**
     * The caller must pass in a Class&lt;? extends Attribute&gt; value.
     * This method first checks if an instance of that class is 
     * already in this AttributeSource and returns it. Otherwise a
     * new instance is created, added to this AttributeSource and returned. 
     */
    public <A extends Attribute> A addAttribute(Class<A> attClass) {
        AttributeImpl attImpl = attributes.get(attClass);
        if (attImpl == null) {
            if (!(attClass.isInterface() && Attribute.class.isAssignableFrom(attClass))) {
                throw new IllegalArgumentException("addAttribute() only accepts an interface that extends Attribute, but " + attClass.getName() + " does not fulfil this contract.");
            }
            addAttributeImpl(attImpl = this.factory.createAttributeInstance(attClass));
        }
        return attClass.cast(attImpl);
    }

    /** Returns true, iff this AttributeSource has any attributes */
    public boolean hasAttributes() {
        return !this.attributes.isEmpty();
    }

    /**
     * The caller must pass in a Class&lt;? extends Attribute&gt; value. 
     * Returns true, iff this AttributeSource contains the passed-in Attribute.
     */
    public boolean hasAttribute(Class<? extends Attribute> attClass) {
        return this.attributes.containsKey(attClass);
    }

    /**
     * The caller must pass in a Class&lt;? extends Attribute&gt; value. 
     * Returns the instance of the passed in Attribute contained in this AttributeSource
     * 
     * @throws IllegalArgumentException if this AttributeSource does not contain the
     *         Attribute. It is recommended to always use {@link #addAttribute} even in consumers
     *         of TokenStreams, because you cannot know if a specific TokenStream really uses
     *         a specific Attribute. {@link #addAttribute} will automatically make the attribute
     *         available. If you want to only use the attribute, if it is available (to optimize
     *         consuming), use {@link #hasAttribute}.
     */
    public <A extends Attribute> A getAttribute(Class<A> attClass) {
        AttributeImpl attImpl = attributes.get(attClass);
        if (attImpl == null) {
            throw new IllegalArgumentException("This AttributeSource does not have the attribute '" + attClass.getName() + "'.");
        }
        return attClass.cast(attImpl);
    }

    private State getCurrentState() {
        State s = currentState[0];
        if (s != null || !hasAttributes()) {
            return s;
        }
        State c = s = currentState[0] = new State();
        final Iterator<AttributeImpl> it = attributeImpls.values().iterator();
        c.attribute = it.next();
        while (it.hasNext()) {
            c.next = new State();
            c = c.next;
            c.attribute = it.next();
        }
        return s;
    }

    /**
     * Resets all Attributes in this AttributeSource by calling
     * {@link AttributeImpl#clear()} on each Attribute implementation.
     */
    public void clearAttributes() {
        for (State state = getCurrentState(); state != null; state = state.next) {
            state.attribute.clear();
        }
    }

    /**
     * Captures the state of all Attributes. The return value can be passed to
     * {@link #restoreState} to restore the state of this or another AttributeSource.
     */
    public State captureState() {
        final State state = this.getCurrentState();
        return (state == null) ? null : (State) state.clone();
    }

    /**
     * Restores this state by copying the values of all attribute implementations
     * that this state contains into the attributes implementations of the targetStream.
     * The targetStream must contain a corresponding instance for each argument
     * contained in this state (e.g. it is not possible to restore the state of
     * an AttributeSource containing a TermAttribute into a AttributeSource using
     * a Token instance as implementation).
     * <p>
     * Note that this method does not affect attributes of the targetStream
     * that are not contained in this state. In other words, if for example
     * the targetStream contains an OffsetAttribute, but this state doesn't, then
     * the value of the OffsetAttribute remains unchanged. It might be desirable to
     * reset its value to the default, in which case the caller should first
     * call {@link TokenStream#clearAttributes()} on the targetStream.   
     */
    public void restoreState(State state) {
        if (state == null)
            return;

        do {
            AttributeImpl targetImpl = attributeImpls.get(state.attribute.getClass());
            if (targetImpl == null) {
                throw new IllegalArgumentException("State contains AttributeImpl of type " + state.attribute.getClass().getName() + " that is not in in this AttributeSource");
            }
            state.attribute.copyTo(targetImpl);
            state = state.next;
        } while (state != null);
    }

    @Override
    public int hashCode() {
        int code = 0;
        for (State state = getCurrentState(); state != null; state = state.next) {
            code = code * 31 + state.attribute.hashCode();
        }
        return code;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }

        if (obj instanceof AttributeSource) {
            AttributeSource other = (AttributeSource) obj;

            if (hasAttributes()) {
                if (!other.hasAttributes()) {
                    return false;
                }

                if (this.attributeImpls.size() != other.attributeImpls.size()) {
                    return false;
                }

                // it is only equal if all attribute impls are the same in the same order
                State thisState = this.getCurrentState();
                State otherState = other.getCurrentState();
                while (thisState != null && otherState != null) {
                    if (otherState.attribute.getClass() != thisState.attribute.getClass() || !otherState.attribute.equals(thisState.attribute)) {
                        return false;
                    }
                    thisState = thisState.next;
                    otherState = otherState.next;
                }
                return true;
            } else {
                return !other.hasAttributes();
            }
        } else
            return false;
    }

    /**
     * Returns a string representation of the object. In general, the {@code toString} method
     * returns a string that &quot;textually represents&quot; this object.
     *
     * <p><b>WARNING:</b> For backwards compatibility this method is implemented as
     * in Lucene 2.9/3.0. In Lucene 4.0 this default implementation
     * will be removed.
     *
     * <p>It is recommeneded to use {@link #reflectAsString} or {@link #reflectWith}
     * to get a well-defined output of AttributeSource's internals.
     */
    // TODO: @deprecated remove this method in 4.0
    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder().append('(');
        if (hasAttributes()) {
            for (State state = getCurrentState(); state != null; state = state.next) {
                if (sb.length() > 1)
                    sb.append(',');
                sb.append(state.attribute.toString());
            }
        }
        return sb.append(')').toString();
    }

    /**
     * This method returns the current attribute values as a string in the following format
     * by calling the {@link #reflectWith(AttributeReflector)} method:
     * 
     * <ul>
     * <li><em>iff {@code prependAttClass=true}:</em> {@code "AttributeClass#key=value,AttributeClass#key=value"}
     * <li><em>iff {@code prependAttClass=false}:</em> {@code "key=value,key=value"}
     * </ul>
     *
     * @see #reflectWith(AttributeReflector)
     */
    public final String reflectAsString(final boolean prependAttClass) {
        final StringBuilder buffer = new StringBuilder();
        reflectWith(new AttributeReflector() {
            public void reflect(Class<? extends Attribute> attClass, String key, Object value) {
                if (buffer.length() > 0) {
                    buffer.append(',');
                }
                if (prependAttClass) {
                    buffer.append(attClass.getName()).append('#');
                }
                buffer.append(key).append('=').append((value == null) ? "null" : value);
            }
        });
        return buffer.toString();
    }

    /**
     * This method is for introspection of attributes, it should simply
     * add the key/values this AttributeSource holds to the given {@link AttributeReflector}.
     *
     * <p>This method iterates over all Attribute implementations and calls the
     * corresponding {@link AttributeImpl#reflectWith} method.</p>
     *
     * @see AttributeImpl#reflectWith
     */
    public final void reflectWith(AttributeReflector reflector) {
        for (State state = getCurrentState(); state != null; state = state.next) {
            state.attribute.reflectWith(reflector);
        }
    }

    /**
     * Performs a clone of all {@link AttributeImpl} instances returned in a new
     * {@code AttributeSource} instance. This method can be used to e.g. create another TokenStream
     * with exactly the same attributes (using {@link #AttributeSource(AttributeSource)}).
     * You can also use it as a (non-performant) replacement for {@link #captureState}, if you need to look
     * into / modify the captured state.
     */
    public AttributeSource cloneAttributes() {
        final AttributeSource clone = new AttributeSource(this.factory);

        if (hasAttributes()) {
            // first clone the impls
            for (State state = getCurrentState(); state != null; state = state.next) {
                clone.attributeImpls.put(state.attribute.getClass(), (AttributeImpl) state.attribute.clone());
            }

            // now the interfaces
            for (Entry<Class<? extends Attribute>, AttributeImpl> entry : this.attributes.entrySet()) {
                clone.attributes.put(entry.getKey(), clone.attributeImpls.get(entry.getValue().getClass()));
            }
        }

        return clone;
    }

    /**
     * Copies the contents of this {@code AttributeSource} to the given target {@code AttributeSource}.
     * The given instance has to provide all {@link Attribute}s this instance contains. 
     * The actual attribute implementations must be identical in both {@code AttributeSource} instances;
     * ideally both AttributeSource instances should use the same {@link AttributeFactory}.
     * You can use this method as a replacement for {@link #restoreState}, if you use
     * {@link #cloneAttributes} instead of {@link #captureState}.
     */
    public final void copyTo(AttributeSource target) {
        for (State state = getCurrentState(); state != null; state = state.next) {
            final AttributeImpl targetImpl = target.attributeImpls.get(state.attribute.getClass());
            if (targetImpl == null) {
                throw new IllegalArgumentException("This AttributeSource contains AttributeImpl of type " + state.attribute.getClass().getName() + " that is not in the target");
            }
            state.attribute.copyTo(targetImpl);
        }
    }

}
